Answer
Nitrogen dioxide.
Work Step by Step
It is named according to the rules of naming binary molecular compounds. A prefix is used to name according to the number of atoms present and the name of the second element ends with '-ide'. Thus, in $NO_{2}$, one atom of nitrogen is present (the prefix mono is not added to the first element in a compound) and two atoms of oxygen are present (Dioxide). Thus, Nitrogen dioxide.
